1. Field of the Invention
The present invention relates to optical transmission systems in general and in particular to an optical transmission system comprising a method and apparatus for cancelling noise, increasing the stimulated Brillouin scattering (SBS) threshold in a standard single mode fiber, and using an external modulator in combination with a distributed feedback (DFB) laser and an erbium doped fiber amplifier (EDFA) to transmit 1550 nanometer (nm) light over standard single mode fiber with a dispersion null at 1310 nm.
2. Description of the Related Art
Conventional prior known optical transmission systems comprising lasers, optical amplifiers, fiber optic cables, and the like, are found to have a number of deficiencies. For example, one of the deficiencies which has been found in prior known optical transmission systems is that the optical radiation from a light source comprising a distributed feedback (DFB) laser and an erbium doped fiber amplifier (EDFA) may be too noisy for some applications.
Another deficiency in prior known optical transmission systems, specifically those comprising a directly modulated DFB and an EDFA as described above which are coupled to a standard single mode fiber with a dispersion null at 1310 nm, is that the EDFA is operable only at wavelengths longer than 1310 nm, e.g. 1550 nm, and that at those wavelengths composite second order (CSO) distortions are generated in the fiber. The standard single mode fiber with a dispersion null at 1310 nm referred to herein corresponds to the SMF-28 fiber which is commercially available from Corning Incorporated, Corning, N.Y.
Heretofore, the methods and apparatus used to reduce noise and compensate for CSO distortions has been limited generally to electrical and optical compensation circuits such as disclosed in applicants' U.S. Pat. No. 5,115,440, applicants' U.S. patent application Ser. No. 07/787,830, both of which are assigned to the assignee of the present application, and a paper presented by Richard B. Childs and Vincent A. O. Byrne in the OPTICAL FIBER COMMUNICATION CONFERENCE, 1990 Technical Digest Series, Vol. 1, Conference Edition, January 1990.
Another deficiency in prior known optical transmission systems has to do with the effect of stimulated Brillouin scattering in a standard single mode fiber.
Stimulated Brillouin scattering (SBS) is a phenomenon which restricts the amount of optical power which can be transferred in a standard single mode fiber with a dispersion null at 1310 nm. That is to say, the SBS threshold, i.e. the maximum amount of optical power which can be transferred in a standard single mode fiber, is limited to a certain amount of power at a given wave length in a certain bandwidth for a given length of the fiber such that, as the length of the fiber or the wavelength increases, its ability to handle the power decreases. For example, the amount of optical power at 1310 nm that can be transferred in a standard single mode fiber with a dispersion null at 1310 nm, 50 kilometers (km) long is approximately 5 milliwatts (mW). At 1550 nm, the amount of power that can be input to the same fiber is approximately 4 mW.
Still another deficiency noted in prior known conventional optical transmission systems arises from the use of yttrium aluminum garnet (YAG) and yttrium lanthanum fluoride (YLF) lasers with standard nonlinear external modulators. Among the problems in these types of systems is the problem that nonlinear external modulators have a relatively low depth of modulation, e.g. 2%, thus providing a relatively low signal/carrier-to-noise ratio, and the problem that the YAG and YLF lasers, while providing relatively high power, e.g. 175 mW and 100 mW, respectively, are relatively expensive and have relatively short life times.